The b'->tW Analysis Working Page

Introduction: Physics and Signatures

The Standard Model (SM) with three generation of quarks has successfully explained most of the physics phenomena up to date. The fourth generation searches cooled off since the limit on the neutrino flavors, and the precise measurement on the electroweak parameters seems to be against the possibility of an extra fermion family. However, it does not guarantee the nonexistence of fourth generation of quarks. The SM is not yet a complete theory, and the options for new physics phenomena are still open.

High energy frontier experiments, such as CDF and D0 at the Tevatron, have performed searches for the fourth generation quarks. CDF has announced their result for seaching a b' quark decaying to tW final state. The study consists of searching for same-sign dilepton pairs based on a data sample of 2.7 fb^-1. The observed events in data (two events) are consistent with the background expectation of 1.9 events. The best limit on the b' mass is 325 GeV/c^2 at 95\% confidence level.

According to different assumptions of the relative strength of |Vcb'| and |Vtb'|, both b' -> cW and b' -> tW(*) could be the major b' decays if m(b') is less than m(t) + m(W) ~ 255 GeV/c^2. For the case of m(b') > 255 GeV/c^2, the decay b' -> tW is open and expected to be the dominant channel. The transition b' -> cW would be suppressed for such a heavy b' scenario unless with a large Vcb'. The decay b'-> t' should be kinematically suppressed [if m(t) < m(t') < m(b')], and b' -> b modes are only proceed through the second order loop diagrams.

When both b' quarks decay into top quark and W boson, it becomes a four W final state. The basic signature is bbWWWW for the pp -> b'b' production. Each W boson can decay through either leptonic W -> l nu or hadronic W -> di-jet. We order the possible final states according to the number of leptonic W's as follows:

  • Mono-leptonic b'b' -> Wlnu Wjj Wjj Wjj + bb:
If we consider that the W bosons from b' decays decay hadronically, such events are practically tt + 4 jets events. Since there is only one neutrino in the final state, one can fully reconstruct the events with the missing energy measurement.

  • Dileptonic b'b' -> Wlnu Wlnu Wjj Wjj + bb:
We categorize the dileptonic channels into two signatures, one with opposite-sign leptons and the other one with same-sign leptons in the final state. The former one contains three topologies: prompt W -> l nu decays (fully hadronic ttbar), dileptonic ttbar, or a fully dileptonic b' decays (while the other b' decays fully hadronically). Only the last topology has a b' quark that can be fully reconstructed for the mass measurement. The signature of same-sign leptons is unique and the backgrounds from the Standard Model are negligibly small. It is only produced through a leptonic top and a leptonic W, but they are not decaying from the same b' quark. Thus, the b' quark cannot be fully reconstructed for same-sign dileptonic events.

  • Trileptonic and quadrileptonic b'b' -> Wlnu Wlnu Wlnu Wjj + bb and b'b' -> Wlnu Wlnu Wlnu Wlnu + bb:
The sub-branching ratios for the trileptonic mode and the quadleptonic mode are much smaller comparing to the former two categories. Both modes have very unique decay signatures and free from most of the SM backgrounds. The b' quark cannot be reconstructed for all of these modes.

This effort is running under the Exotica physics group:

Related Participants

Institution Related People Remark
National Taiwan University Kai-Feng (Jack) Chen*, Rong-Syang Lu, Yen-Ming Tzeng, Jui-Te Wei, Yeong-Jyi Lei, Kai-Yi Kao, Yu-Wei Chang, Paolo Bartalini, Wei-Shu (George) Hou *Contact person
Rutgers University Steve Schnetzer, Dmitry Hits  

Document & Presentations

10 TeV analysis (EXO-09-012):

General remark: A "minimum" update to the previous 14 TeV analysis (EXO-08-009). Since the 300 GeV/c^2 b'->rW has been already excluded by CDF and the initial energy of LHC will not reach 14 TeV, thus this analysis is carried out with the following two updates:

  • Instead of 100/pb integrated luminosity at 14 TeV, this update study targets an initial data set of 200/pb at 10 TeV cm energy.
  • The default reference signal is 400 GeV/c^2 b'->tW (instead of 300 GeV as in the previous analysis).

14 TeV analysis (EXO-08-009):

  • The internal note for the initial analysis, working with 1_3_X samples (obsolete): CMS AN-2008/042.

Analysis Codes

The PAT embedded analysis code, now it's working with CMSSW_2_2_X (X<=9 as PAT version 1). Current version running under 2_2_9 together with the PAT patches (See SWGuidePATRecipes for 2_2_X PAT version 1). The current working code can be found at /afs/ with a single source code, build file, and a testing configuration file.

  • "BprimeAnalyzer" by R.-S. Lu:
The analysis code works for CMSSW_1_6_X series. It produce a general format root tree for b'b -> multi-W analysis.

  • "Heavybprime" (this is obsolute.):
The original analysis code for CMSSW_1_3_X series analysis. It produce simple raw-wise ntuples for the further studies. The code can be found at /afs/

Monte Carlo Samples

10 TeV Samples (2_1_X and 2_2_X)

Process Cross-section (pb) Size Equ. Lumi (pb^-1) Generator Simulator Remark
bprime, M(b') = 300 GeV/c^2 13.6(LO) 53.4K 3,930 Pythia Full  
bprime, M(b') = 400 GeV/c^2 2.80(LO) 40.8K 14,600 Pythia Full  
bprime, M(b') = 500 GeV/c^2 0.78(LO) 40.8K 52,300 Pythia Full  
tt+jets 414(NLO) 1.03M 2,490 Madgraph Full  
tt+W(+j) 0.33(LO) 8K 24,200 Madgraph Full  
tt+Z(+j) 0.32(LO) 8K 25,000 Madgraph Full  
ttW^+W^- 0.0035(LO) 4K 1,140,000 Madgraph Full  
W+jets 40,000(LO) 10.3M 258 Madgraph Full  
Z+jets 3,700(LO) 1.2M 324 Madgraph Full  
W+jets 40,000(LO) 101.3M 2530 Madgraph Fast  
Z+jets 3,700(LO) 9.7M 2630 Madgraph Fast  
WW inclusive 74.0(LO) 204K 2,760 Pythia Full  
ZZ inclusive 10.5(LO) 201K 19,100 Pythia Full  
WZ inclusive 32.0(LO) 249K 7,780 Pythia Full  
same-sign WW+jj 0.28(LO) 6K 21,400 Madgraph Full  
QCD (5 flavors, HT: 100-250 GeV) 15,000,000(LO) 14.2M 0.95 Madgraph Full  
QCD (5 flavors, HT: 250-500 GeV) 400,000(LO) 5.4M 13.5 Madgraph Full  
QCD (5 flavors, HT: 500-1000 GeV) 14,000(LO) 4.9M 350 Madgraph Full  
QCD (5 flavors, HT: 1000-inf GeV) 370(LO) 1.0M 2,700 Madgraph Full  

14 TeV Samples (1_8_4)

General status: The CMSSW_1_8_4 fastsim samples are in processing and quality checking. By default no pile-up mixing included for the samples. Known problems: The charges for the candidates in the genParitlceCandidates collection are wrong. The GenParticleCandidate2GenParticleProducer is slightly modified to fix them in genParticles collection (genParitlceCandidates remain untouched). For the private produced MC, the HepMCProduct collection is kept for the PDF information.

No. of events left = the number of events passing dilepton filtering (generic 2e, 2mu, or e+mu regardless of the charge, pt>10).

Physics Process Generator Simulation Cross-section Amount Status Remark
b'b'->t(^*)W(^*)t(^*)W(^*), m(b')=200 GeV pythia6 1_8_4 fastsim 262 pb 40K done. / 22479 events left cfi
b'b'->tWtW, m(b')=300 GeV pythia6 1_8_4 fastsim 34.9 pb 40K done. / 22329 events left cfi
b'b'->tWtW, m(b')=400 GeV pythia6 1_8_4 fastsim 8.05 pb 40K done. / 23907 events left cfi
b'b'->tWtW, m(b')=500 GeV pythia6 1_8_4 fastsim 2.45 pb 40K done. / 24156 events left cfi
b'b'->tWtW, m(b')=300 GeV, with low lumi. pile-ups pythia6 1_8_4 fastsim 34.9 pb 40K done. / 21755 events left
ttW comphep 1_8_4 fastsim 0.65 pb 6K done. / 2371 events left
ttW+j (pT>10 GeV) comphep 1_8_4 fastsim 1.23 pb 6K done. / 2566 events left
ttZ comphep 1_8_4 fastsim 1.01 pb 6K done. / 2466 events left
ttZ+j (pT>10 GeV) comphep 1_8_4 fastsim 2.40 pb 6K done. / 2619 events left
ttWW comphep 1_8_4 fastsim 0.016 pb 4K done. / 2175 events left
ttH, H->WW m(H) = 140 GeV pythia6 1_8_4 fastsim 0.168 pb 4K done. / 2098 events left cfi
ttH, H->WW m(H) = 160 GeV pythia6 1_8_4 fastsim 0.210 pb 4K done. / 2152 events left cfi
ttH, H->WW m(H) = 180 GeV pythia6 1_8_4 fastsim 0.150 pb 4K done. / 2230 events left cfi
tt+nj madgraph 1_8_4 fastsim 694 pb 1026.5K done. / 299795 events left
Z+nj, Z->ee,mumu madgraph 1_8_4 fastsim 7000 pb 1730K done. / 499935 events left
W+nj, W->enu, munu, taunu madgraph 1_8_4 fastsim 60000 pb 4409.8K done. / 57532 events left
ZZ inclusive pythia6 1_8_4 fastsim 15.5 pb 50K done. / 6211 events left cfi
WZ inclusive pythia6 1_8_4 fastsim 51.5 pb 50K done. / 4460 events left cfi
bb+nj, HT in [100, 250] GeV madgraph 1_8_4 fastsim 900000 pb 10979K done. / 362907 events left
bb+nj, HT in [250, 500] GeV madgraph 1_8_4 fastsim 50000 pb 800K done. / 89461 events left
bb+nj, HT in [500, 1000] GeV madgraph 1_8_4 fastsim 4000 pb 60K done. / 10705 events left
bb+nj, HT >1000 GeV madgraph 1_8_4 fastsim 150 pb 21.63K done. / 4671 events left
QCD pt 20 - 30 GeV pythia6 1_8_4 fastsim 630000000 pb 1068K done. / 360 events left
QCD pt 30 - 50 GeV pythia6 1_8_4 fastsim 163000000 pb 960K done. / 2826 events left
QCD pt 50 - 80 GeV pythia6 1_8_4 fastsim 21600000 pb 296K done. / 5013 events left
QCD pt 80 - 120 GeV pythia6 1_8_4 fastsim 3080000 pb 289K done. / 12571 events left
QCD pt 120 - 170 GeV pythia6 1_8_4 fastsim 494000 pb 54K done. / 3457 events left
QCD pt 170 - 230 GeV pythia6 1_8_4 fastsim 101000 pb 55K done. / 3998 events left
QCD pt 230 - 300 GeV pythia6 1_8_4 fastsim 24500 pb 51K done. / 3885 events left
QCD pt 300 - 380 GeV pythia6 1_8_4 fastsim 6240 pb 49K done. / 3739 events left
QCD pt 380 - 470 GeV pythia6 1_8_4 fastsim 1780 pb 49K done. / 3899 events left
QCD pt 470 - 600 GeV pythia6 1_8_4 fastsim 683 pb 58K done. / 4500 events left
QCD pt 600 - 800 GeV pythia6 1_8_4 fastsim 204 pb 60K done. / 4864 events left
QCD pt 800 - 1000 GeV pythia6 1_8_4 fastsim 35.1 pb 57K done. / 4363 events left

  • Other stuff:
    • Configuration files for CSA07 production can be found at: /afs/

Tasks and Issues

Topic Short comment People
Trigger which trigger streams that we want to use (for the early discovery)  
Muon Muon selectons and isolation  
Electron Electron selectons and isolation  
Charge mis-ID rate how to measure this number from data  
Jets Jet selecton (algorithms, threshold)  
  Jet counting (how to examine this from data)  
  Effects of pile-ups  
Event selecton What are the variables to be used, thresholds  
  How to estimate the background level from data  
Cross-section extraction Adopting the counting experiment  
  Systematic uncertainties  
  Significance calculations  

Useful Links

-- KaiFengChen - 14 Jun 2008

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Topic revision: r21 - 2009-06-05 - KaiFengChen
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